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Slashterix
Asked: 2011-11-19 15:48:15 +0800 CST

Using watch to run a shell script - but the script goes defunct

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So I want to keep an eye on my network connectivity so I can make a report for my ISP. (seems like the modem goes out for about 30-60 seconds about once a day)

So I copied this handy little script

http://www.cyberciti.biz/tips/simple-linux-and-unix-system-monitoring-with-ping-command-and-scripts.html

And an running it with 

watch -n 15 ./ping-test.sh

I have been testing it by pulling the plug on my modem.

The first time watch runs the script after the network goes out it runs fine (sends some email) and then goes defunct. Once networking is restored the script "finishes" and then watch runs it again.

So I'm getting mail for when the network goes down, but not how long it was down for.

If I run the script straight from the command line it runs (emails) and then exits.

Any ideas why? How can I get the script to keep running once the network is down?

linux bash ping watch

4 Answers

  1. You can do this in realtime by using "ip monitor". This will output any events such as the ip address being removed or added the link state changing from operational to disconnected, or a route changing in some way.

    ip monitor all >> /some/log.log &

    You must open in append mode. If you want to leave it to run outside of the terminal you can just disown the job with "disown %1"

    If you need timestamps too, theres a couple of ways. An easy way is to use inotifywait to check if the file gets an update, then append your date to it. Itd be easy to do the above and this into a shell script and perhaps disown that instead so you get

    ip monitor all >> /some/log.log &
while inotifywait -qq -e modify /some/log.log; do 
  sleep 0.2;
  date >>/root/mon.log; 
done

    Enjoy

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  2. Your solution with 'watch' command is not good. I would try to solve this with something like this:

    #!/bin/bash
HOSTS="foo.bar foo2.bar"
COUNT=4
while :
do
    for myHost in $HOSTS
    do
      count=$(ping -c $COUNT $myHost | grep 'received' | awk -F',' '{ print $2 }' | awk '{ print $1 }')
      if [ $count -eq 0 ]; then
        # 100% failed 
        echo "Host : $myHost is down (ping failed) at $(date)"
      fi
      done
sleep 15 # here is your refresh rate
done

    and then run it as normal script: ./script_name.sh

    terminate with CTRL+C.

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  3. Best Answer

    ping will hang if it cannot reach its target and that maybe why the script goes defunct because the ping is still waiting for a reply. Try adding the -w deadline or -W timeout option to the ping.

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  4. It goes defunct because it is already finished but depends on a parent process - watch command which is still alive.

    So I'm getting mail for when the network goes down, but not how long it was down for.

    That script is already print the date when the remote system is down:

    echo "Host : $myHost is down (ping failed) at $(date)" | mail -s "$SUBJECT" $EMAILID

    How can I get the script to keep running once the network is down?

    It is pinging with -c 1 so it will stop after sending 1 ECHO_REQUEST packet.

    Put it in cron to run every minutes:

    */1 * * * * /path/to/your/script
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